820-3115 no cpu vcore

[Kyle93]

Have a 820-3115 that mild water damage but turns on with 0V cpu vcore.

Already replaced u7400, Q7510 and Q7520

r7402 and r7403 are good

ALL_SYS_PWRGD 3.2V

Pin1 of u7400 12.41V

what to try or check next?

 

[2informaticos]

Do not detect any drop pulse at TONA/B at least?
Check all SPI termination resistors and traces around U6100.
You can also try new BIOS.
Can you see if U2600 sends out 25MHz at pin 9, or if tries at least?

 

[Kyle93]

No drop voltage at TONA/B. A is steady 1.745v B is 12.43v

Termination resistors and traces look good around u6100

Already tried new bios.

Do not have oscilloscope to measure MHZ but I am getting .7771V from pin 9 of u2600

 

[2informaticos]

"A is steady 1.745v"
Bad R7402 (higher value), or bad U7400...

 

[Kyle93]

Replaced R7402 and replaced U7400 twice now and voltage on A starts at 1.886 but continuously drops the longer the machine runs

 

[Kyle93]

Also need to mention even though i have .5V at cpu vcore, the cpu is still getting warm

 

[2informaticos]

TONA voltage is completely unusual.
Check all resistors and traces around U7400.
Does it get 5V at pins 19, 29, 46?
3V present at pin 47?
Also check all its MOSFETs; replace them, just in case.

 

[Kyle93]

resistors and traces look fine around u7400

5V on pin 19, 29. Only 3.6V on pin 46

3v Present at pin 47

Replaced all mosfets, still same problem

 

[Kyle93]

TON A voltage is now 12.46V

 

[2informaticos]

"Only 3.6V on pin 46"
Are you sure?
Check R/C7401.

 

[Kyle93]

Yes I am sure, just checked again. pin 46 3.6V

R/C7401 both 3.6 V

 

[2informaticos]

"Check R/C7401."
R7401=x ohm???
Voltage goes up if remove C7401?

 

[Kyle93]

r7401 10.7

voltage went up to 3.7 after removal of c7401 on pin 46

 

[2informaticos]

Pin46 should have 5V too.
Change U7400...

 

[Kyle93]

Changed u7400 again. I now have 5v on pin 46 and 1.14v at cpu vcore when it first starts up, however I have a quarter fan spin issue now.

 

[2informaticos]

Try different RAM module, one at a time, each slot.
VCORE should drop to aprox 0.8V when board works.
Test if board starts in SMC bypass mode.

 

[Kyle93]

Board somehow lost cpuvcore again since I last tested. Currently have 5V at pin 46 but 0V on cpu vcore again. What to check for this?

 

[Kyle93]

cpu vcore is back along with the quarter fan spin.

cpu vcore does drop to .889V before the fan stops spinning

Board starts and stays running while in SMC bypass mode but 0V on cpu vcore while in SMC bypass mode

 

[2informaticos]

Check R7051/52...

 

[Kyle93]

R75051 and R7052 12.60V